This is a npm module that gets a CurrentUser session based on a login to Google. I struggled with just using a plain jQuery.ajax style interface to this and ended up using googleapis interface instead.

var GoogleLogin = require('google-login');
var oGoogleLogin = new GoogleLogin();
// `/login` is the place to login
app.get('/login', jQuery.proxy(oGoogleLogin.login, oGoogleLogin));
//don't need to proxy this because the currentUser is in the session
app.get('/logout', oGoogleLogin.logout);
app.get('/currentUser', oGoogleLogin.currentUser);
//redirect to login page unless we have a session with a current user
app.use(function(req, res, next){
if((req._parsedUrl.path == '/' || req._parsedUrl.path.indexOf('html') != -1)
&& (!req.session || !req.session.profile))
{
// then we are going for an html file without logged in user
req._parsedUrl.path = '/login.js.html';
renderAsync.renderAsync(req, res);
}else{
next();
}
});